1. INTRODUCING TIDAL DWARF GALAXIES

There is yet no consensual definition of a Tidal Dwarf Galaxy
(TDG). Let's however stick to the acronym to define it:

Tidal refers to an object made of material that was tidally
expelled from galaxies. The definition may be a bit enlarged and
includes objects born in general in debris of galaxy-galaxy collisions:
tidal tails of mergers, but also collisional rings and even gas stripped
in the intergalactic medium by other processes than tidal forces. The
key element is that the building material of TDGs used to belong to a
larger parent galaxy and was thus pre-enriched. In practice late-type,
rotating colliding galaxies generate more debris than early-type,
dynamically hot galaxies. As a consequence, TDGs should mostly be
produced by wet mergers involving spiral galaxies.

Dwarf means that the object born in tidal tails should
have the size and mass of a dwarf galaxy - although this is a loose
criterion given the large range of sizes/masses exhibited by dwarf
galaxies (from the ultra faint ones around the Milky Way to the
Magellanic type objects). This specification allows to disentangle TDGs
from the Super Star Clusters and other compact stellar objects that are
formed as well in colliding systems.

Galaxy implies that the
system is kinematically decoupled from its parent galaxy and
gravitationally bound. This increases its ability to survive external
gravitational stirring or internal destructive processes such as stellar
feedback. In other words, TDGs are not transient objects but correspond
to genuine condensations of matter that have collapsed in-situ within
collisional debris.

How does this definition of a TDG translate into observational properties?

Being recycled objects, TDGs have
inherited from their parents the metal content of their interstellar
medium. Thus their metallicity tells about the past chemical enrichment
of their parents, and is thus not correlated with their actual mass,
contrary to conventional galaxies. Made out of pre-enriched material,
they should have an excess of heavy elements, provided that their
parents were themselves metal rich. This implies as well that their dust
content and molecular gas content, as traced by CO, is higher than in
regular star-forming dwarf galaxies.

Made out of material
expelled from the dark-matter poor disks of their parent galaxies, TDGs
have accreted little of their dark-matter content. As a consequence,
their luminous mass (stars and gas) should be close to their dynamical,
total, mass, contrary to conventional dark-matter dominated galaxies
(but see Section 3).

Examples of observed Tidal Dwarf Galaxies are shown in
Fig. 1. On these images of colliding systems,
the TDGs appear as red stains on blue ribbons, i.e. star-forming objects
within gas-rich tails. The most massive of them are usually located near
their tip. Several papers have exploited the rich Ultraviolet/GALEX
Infrared/Spitzer databases on interacting galaxies and investigated in
details how star-formation proceeds in collisional debris (e.g.
[27,
25,
2]).
At this stage, it is worthwhile noting that the vast majority of the
Tidal Dwarf Galaxies so far securely identified are young objects,
formed in mergers that occurred less than one Gyr ago. They still
exhibit the umbilical cord linking them to their parents... i.e. the
tails and bridges in which they were formed have not had the time to
evaporate. Once evolved, TDGs should become undistinguishable from
regular satellite galaxies on optical images.

Figure 1. Sample of colliding systems
exhibiting TDG candidates. The distribution of the gas is shown in blue
and the star-forming regions in red.